Discharge tube



1933. K. M, VAN GESSEL. ET AL 1,937,735

DI SCHARGE TUBE Filed Aug. 23, 1928 lNVENTOR #4262 1/ 144A 65555.4

ATTO R N EY Patented Dec. 5, 1933 PATENT OFFICE 4 DISCHARGE TUBE Karel Marinus Van Gessel, Menze Bandringa, and

Cornelis Hendrik Morel, Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application August 23, 1928, Serial No. 301,534, and in the Netherlands September 15, 1927 2 Claims.

The invention relates to a discharge tube comprising an indirectly heated cathode. Especially when used in sets for wireless telegraphy or telephony, discharge tubes with such cathodes ,5; are suitable to be fed with alternating current.

Discharge tubes for wireless telegraphy or telephony comprise, as a rule, incandescent cathodes which are heated by a continuous current, for example, with the aid of an accumulator. The use of continuous current for heating the incandescent cathode is recommended for the reason that an alternating current gives rise to the production of humming noises corresponding to the frequency of the alternating current. It is known, however, to use in discharge tubes cathodes fed with alternating current which have not this disadvantage. Suchoathodes may consist of a small tube which is indirectly heated and which in consequence thereof emits itself electrons. This indirect heating can be effected by means of heat radiation or again by means of electron-bombardment. For this purpose, a filament may be arranged in thesaid tube so as to be insulated therefrom, said filament being connected to leading-in wires' which supply the heating current to the filament. Now it has been stated, that during the operation of discharge tubes comprising such cathodes, electrons emanating fromthose parts of the filament which protrude from thetube, may participate in the discharge and thus may spoil the good properties of the discharge tube. The invention has for its object to obviate the said disadvantage.

According to the invention, meansare provided to prevent electrons emanating from the heating member to participate in the. discharge. Preferably, the cathode consists of a small tube surrounded, at least at one of its ends, by a screen which forms a separating surface between the discharge path and the part of the heating memberwhich protrudes from the tube. The tube should preferably be coatedwith electro-active substances capable of emitting electrons under the influence of the high temperature of the tube. The tube may be heated by heat-conduction or by heat-radiation in consequence of the filament provided in the said tube being heated to a high temperature, or by an electron-bombardment which may be effected by heating the filament mounted in the tube and by applying a potential difierence between this filament and the tube. In both cases this filament will be hereinafter referred to as heating wire. Good results have been obtained with a tube which is closed at one of its ends and which contains a loop-shaped heating wire which is insulated at its surface and the two ends of which emerge from the other open end of the tube. Use should preferably be made of a seamless tube. The screen surrounding the cathode may be given a determined potential, for example, that of the cathode; the screen may also be connected to a leading-in wire specially provided for this purpose so that from outside the discharge tube a suitable potential can be applied to the screen.

The invention will be more clearly understood by referring to the accompanying drawing which represents, by way of example, an embodiment of the invention. In the drawing:

Fig. 1 is a front elevation of the stem of a discharge tube according to the invention, the grid being omitted for clearness, and

Figs. 2 and 3 are respectively a side elevation and a plan of such a discharge tube.

In these figures, a cathode is denoted by 1, a grid by 2 and an anode by 3. The cathode is constituted by a narrow tube consisting, for example of nickel. First this tube is copper-plated and then oxidized, the copper oxide layer being finally coated with an alkaline earth, for example, with barium. The heating of such a cathode results in barium oxide being formed. A loopshaped heating wire 4 is so arranged in the tube, that its two ends emerge from the open end 5 while the middle of the heating wire is located close to the closed end 6 of the tube 1. The heating wire is connected to pole wires 7 and 8, which permit the wire to be heated. The tube 1 is connected to a pole wire 9 which is taken out of the discharge tube. Around the cathode is arranged a screen 10 which forms a separating surface between the discharge path and that part of the heating member 4 which protrudes from the tube. The screen is connected to a pole wire 11 and can consequently be connected to a point of a suitable potential. Preferably, however, the screen is connected within the discharge tube to the cathode 1. The grid 2 is carried on supports 12, one of which acts as a pole wire. The filament 4 is coated with an insulating substance so that the two parts of the loop located in the tube 1 cannot give rise to short-circuits. For this purpose, the filament may be coated with a substance such as aluminum oxide, silicit acid or the like.

It has been stated that discharge tubes according to the invention, which comprise a cathode as that above described, do not show the disadvantages of an incandescent cathode fed with alternating current.

Having thus described our invention, we claim:

1. A thermionic tube including an elongated tubular cathode closed at one end, an anode surrounding said cathode, said anode being of shorter length than said cathode, indirect heating means for said cathode including a filamentary wire extended therein with its ends protruding from said cathode and means for preventing electrons emanating from the protruding ends of said filamentary wire from reaching said anode including a disc shaped screen between said protruding ends of said Wire and said anode and extending in a plane transverse of said cathode to bring its periphery at all points beyond the periphery of said anode, said screen being out of electrical contact with said cathode, and a terminal for said screen.

trical contact with said sleeve and in a plane between the protruding ends of said heater filament and the adjacent end of said anode and extending transversely of said sleeve to beyond said anode, and a terminal for said screen.

KAREL MARINUS VAN GESSEL. MENZE BANDRINGA. CORNELIS HENDRIX MOREL. 

